Method for p-type doping of semiconductor structures formed of group II and group VI elements

ABSTRACT

A method and apparatus (10) for forming a p-doped layer (68, 80, 92) of Group II and Group VI elements by molecular beam epitaxial process in which a nitrogen dopant is introduced as the layer (68, 80, 92) is being grown. In one embodiment, molecular nitrogen is passed through a plasma generator (46) for converting it to activated nitrogen, and the activated nitrogen is conducted through an elongated guide tube (50) toward the substrate (66) upon which a Group II-Group VI layer (68, 80, 92) is being grown. In one embodiment, an n-type dopant source (72) is also provided, the apparatus (10) being operable for forming electrical devices (86, 88) having successive layers (78, 80, 90, 92, 94) of differing electrical characteristics.

"The U.S. Government has certain rights in this invention pursuant tocontract No. DAAH01-91-C-R017."

FIELD OF THE INVENTION

This invention relates in general to the formation of p-dopedsemiconductor materials and, more particularly, to a method andapparatus for p-type doping and epitaxial growth of semiconductorstructures formed of Group II-Group VI materials.

BACKGROUND OF THE INVENTION

Semiconductor structures having layers formed of Group II-VI compoundsor alloys are of interest in the fabrication of infrared radiationdetectors and imagers. For example, HgTe, and Hg_(1-x) Cd_(x) Te arenarrow bandgap Group II-Group VI semiconductors which may be utilizedfor emitting and absorbing infrared radiation in several regions of theinfrared band. Such materials are often employed in the fabrication ofinfrared focal plane detector arrays, in which they serve asphotodetectors, and in infrared light sources such as lasers, in whichthey serve as emitters. Since both the photodetector and radiationemitter devices require p-n junctions, p-type doping of at least oneGroup II-Group VI layer is necessary in each.

In certain applications, because of the necessity for tailoring suchsemiconductor structures with extremely sharp transitions betweenlayers, the layers are advantageously formed by epitaxial growthprocesses. Molecular beam epitaxy (MBE) is an important techniqueutilized in the formation of device structures requiring very thinlayers, such as double-layer heterojunction photodiodes, or two-colorphotodiodes, in which more than two active layers are required. In suchdevice structures, molecular beam epitaxial growth processes have beenemployed for forming layers of both p-doped and n-doped Group II-GroupVI materials.

MBE techniques are generally known in the art and have been widelydiscussed in the literature. See, for example, the following articles,which are hereby incorporated by reference: A. Y. Cho and J. R. Arthur,in Progress in Solid State Chemistryedited by J. McCaldin and G.Somorjai (Pergamon, N.Y., 1975), Vol. 10, p. 157; L. L. Chang, inHandbook on Semiconductors, edited by S. P. Keller (North-Holland,Amsterdam, 1980), Vol. 3 Chapter 9; C. E. C. Wood, in Physics of ThinFilms, edited by C. Haft and M. Francombe (Academic, N.Y., 1980), Vol.11, p. 35; C. T. Foxon and B. A. Joyce, in Current Topics in MaterialsScience, edited by E. Kaldis (North-Holland, Amsterdam, 1981), Vol. 7,Chapter 1. Articles relating to p-type doping of Group II-Group VIlayers during MBE growth include J. M. Arias, et al., J. Vac. Sci.Technology A8, pp. 1025-1033 (1990), which are also incorporated byreference.

As noted, molecular beam epitaxy provides a number of advantages in theformation of various semiconductor structures. In general, MBE processespermit the growth of thin films with extremely high crystalline quality.Normally in such MBE processes, neutral molecular or atomic species aredirected onto a suitable substrate in a vacuum, the substrate beingheated to a temperature sufficient to permit deposited atoms to movelaterally for average distances of at least several angsttoms to permitthe deposited atoms to find their energetically preferred sites. P-typedoping of Group II-Group VI materials during epitaxial growth processeshas been somewhat difficult, and it has been particularly difficult inthe molecular beam epitaxial growth of mercury-containing layers,because of the high volatility of mercury. Accordingly, relatively lowsubstrate temperatures, e.g., 185° C.-200° C., have been required. Whenit has been attempted to introduce a p-type dopant such as arsenic inassociation with the MBE growth of Group II-Group VI layers,particularly in the formation of p-Hgl_(1-x) Cd_(x) Te, the MBE processhas become further complicated, as will be discussed in the followingsection.

DESCRIPTION OF THE PRIOR ART

P-type doping of Group II-Group VI structures has, in the past, beenaccomplished either by intrinsic doping or by the introduction of GroupV acceptor impurities, such as arsenic, into the structures, Such GroupV elements result in p-type doping of Group II-Group VI materialsbecause they occupy the Group VI lattice vacancies. Devices for whichsuch MBE processes have been suggested in the past include n-on-phomojunction diode structures such as those having a structurecomprising: (1) a CdZnTe substrate, (2) a p-Hg_(1-x) Cd_(x) Te layer,wherein x=0.2, and (3) an nHg_(1-y) Cd_(y) Te layer, wherein y=0.2.Other diode structures include a p-on-n double layer heterojunetion,having: (1) a CdZnTe (4% Zn) substrate, (2) an n-Hg_(1-x) Cd_(x) Te(x=0.2) base layer, and (3) a p-Hg_(1-y) Cd_(y) Te (y=0.3) cap layer.MBE processes are advantageously used particularly when the p-type caplayers must be very thin, e.g., 1.5 μm. However, in prior-art MBEprocesses such as those used for forming double layer heterojunctiondiodes, several limitations have been experienced. As will be understoodby those in the art, such structures require precise vertical alignmentof the p-n junction (e.g., the junction between the p-doped material andthe n-doped material) with the heterojunction (the junction between theHg_(1-x) Cd_(x) Te layer and the Hg_(1-y) Cd_(y) Te layer). In MBEgrowth of Hg_(1-x) Cd_(x) Te, n-type doping is typically accomplished byemploying a dopant such elemental indium, in accordance with processesknown in the art. However, processes for p-type doping of Hg_(1-x)Cd_(x) Te during MBE growth have been less established.

In the past, arsenic, in the form of As₄, was used in p-type doping ofCdTe during MBE growth at substrate temperatures of about 260°-300° C.However, arsenic activation in HgTe and in Hg_(1-x) Cd_(x) Te, which areoften grown at 175°-200° C., has not been satisfactory because thearsenic is not sufficiently active at such low temperatures.Consequently, p-type doping of a device layer of MBE Hg_(1-x) Cd_(x) Tehas required a repetitious MBE growth process, accomplished by: (i)successive MBE growth of a plurality of thin CdTe/HgTe superlatticelayer pairs, in which only the CdTe layer of each pair is doped witharsenic, and (ii) an annealing and interdiffusion process carded out at350° C. or higher, after growth of the multiple CdTe and HgTe layerpairs, to form p-Hg_(1-x) Cd_(x) Te by interdiffusion of the layers tohomogenize arsenic in the resultant Hg_(1-x) Cd_(x) Te structure. Such adiffusion process is effective only when the adjacent CdTe and HgTelayers are less than a few hundred angstroms, and the MBE depositionsteps must thus be repeated many times, e.g., for 100 periods, toprovide a p-doped structure of sufficient thickness for use in a typicaldiode or transistor. The process is thus tedious, time consuming, andcostly.

Additionally, at the interface between an arsenic doped p-Hg_(1-x)Cd_(x) Te layer and an n-doped layer, as in the above-described doublelayer heterojunction diode structure, it is difficult to align the p-njunction with respect to the heterojunction. That is, p-type doping anddiffusion processes utilizing arsenic or other fast diffusing dopantsaffect the "vertical" alignment of the p-n junction with respect to theheterojunction, since such fast-diffusing dopants tend to diffusethrough the heterojunction and into any adjacent layer. This junctionmisalignment or junction-positioning problem results in inferior deviceperformance, since the arsenic may diffuse as much as 2 μm or more fromthe heterojunction into the adjacent layer. The handling of toxicsubstances such as arsenic is, of course, also of concern and entailsadditional safety-related procedures.

Arsenic in the form of a Group II₃ V₂ compound of cadmium and arsenic,of which one example is Cd₃ As₂, has been used for p-type doping ofmolecular beam epitaxial Hg_(1-x) Cd_(x) Te, as described in U.S. Pat.No. 5,028,561. In such a process, a flux is formed from a combination ofa Group V dopant, and a Group II material. The Group II-V combination isapplied with a Group VI element during molecular beam epitaxy growth ofa Group II-VI substrate, wherein the dopant flux occupies and ties upthe normal metal vacancies in the metal lattice, and acts as a p-typedopant. However, such processes entail the disadvantages discussed abovewith respect to junction misalignment when it is required to formprecisely matched heterojunctions and n-p junctions in thin layers ofGroup II-Group VI materials, and they again entail the difficultiesinherent in the use of such highly toxic substances. Thus, processes inwhich Group V dopants such as arsenic, lead, or phosphorus areincorporated in molecular beam epitaxial growth of II-VI materials havein general entailed a number of technical difficulties anddisadvantages.

In contrast, nitrogen, also a Group V element, is a relatively slowdiffuser and is non-toxic in its molecular form (N₂). In the past,however, nitrogen has not been considered acceptable as a p-type dopantfor Group II-VI materials, for the following reasons. Molecular nitrogenis quite stable, having a triple co-valent bond, and its interatomicforces are very high. Because the nitrogen molecule is both very stableand highly symmetrical, intermolecular forces are very small at MBEgrowth temperatures. Whereas arsenic may be introduced in an epitaxialgrowth chamber in its inactive, molecular form, molecular nitrogen mustbe transformed into an active, atomic form if it is to react effectivelywith the Group VI lattice during epitaxial growth. Further, techniquesutilized for the conversion of molecular nitrogen to its atomic formgenerally entail the use of a plasma source for activating the molecularnitrogen. When mercury is one of the epitaxial growth elements, it tendsto vaporize at a high rate when heated during the MBE growth procedure.Thus, if a conventional MBE growth apparatus were used for the epitaxialgrowth of a mercury-containing layer, the mercury vapor would tend toenter and contaminate any components such as a plasma chamber utilizedfor forming activated nitrogen. Additionally, since the plasma isreactive, it could tend to etch the surfaces of the substrate, thenitrogen supply conduits, and other portions of the MBE apparatus, andto result in the introduction of impurities and residues tending tocontaminate the chamber. For these and other reasons, activated nitrogenhas not been considered acceptable as a dopant in such MBE processesand, indeed, the use of such a plasma source is unique, and contrary tothe molecular epitaxial diffusion process, in that the other elementsare being introduced as molecular fluxes derived from heated cruciblesof the materials.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for epitaxialgrowth of a semiconductor structure having Group II and Group VIelements while effecting p-type doping of the semiconductor by theintroduction of an activated nitrogen doping agent into the epitaxiallayer. In one embodiment, the doping agent is introduced into the MBEchamber in the form of an activated nitrogen gas converted frommolecular nitrogen by means of an electron cyclotron resonance apparatusconnected between a source of molecular nitrogen gas and the interior ofthe MBE chamber. The electron cyclotron resonance apparatus is operablefor directing a constant flow of highly activated nitrogen into theevacuated MBE chamber during epitaxial growth of one or more layers of aGroup II-Group VI structure. The epitaxial growth of the p-typesemiconductor layer is accomplished under a high vacuum, at molecularbeam epitaxy pressures and temperatures appropriate for the MBE growthof the particular Group II and Group VI materials. The effective dopantdeposition rates, and therefore, the doping intensity, are controlled byadjusting the flow rate of the nitrogen through the plasma source.Increased flow rates of activated nitrogen result in higher levels ofp-type doping of the resultant layer, and lower levels of doping resultfrom lower flow rates.

In the molecular beam epitaxial growth process, a substrate is supportedwithin a MBE chamber evacuated to about 10⁻¹⁰ torr. The substrate has atleast a surface region having one or more Group II materials, such asmercury and cadmium, or cadmium alone, and the substrate is positionedwith its surface facing one or more effusion cells, the chamber alsobeing in communication with the source of activated nitrogen.

In the formation of a HgCdTe layer, for example, if one Group IIeffusion cell contains mercury, another may contain cadmium. Or, acombination of two effusion cells having mercury and cadmium telluride,respectively, may be employed, along with the activated nitrogen source.A third cell, of tellurium, provides the Group VI element. Thus, an MBEflux is formed from a combination of activated nitrogen, a Group IImaterial, and tellurium as the Group VI material, for the fabrication ofa p-type Hg_(1-x) Cd_(x) Te layer suitable for use in a narrow band-gapinfrared photodetector device.

The flux is applied to the substrate during MBE growth at a pressure ofabout 1×10⁻⁴ Torr to 4×10⁻⁴ Torr. During MBE growth, the Group Vmaterial, i.e., the activated nitrogen, occupies and ties up vacanciesin the Group VI lattice. Accordingly, the activated nitrogen dopant ispermitted to diffuse into the Group VI lattice, the nitrogen undergoingan ongoing and continuous occupation of the Group VI lattice position,wherein it remains as a p-type dopant. The Group II component of theflux preferably dominates the Group VI element.

In accordance with other aspects of the invention, and as will bediscussed hereinbelow, devices having various combinations of p-type andn-type layers may be formed by successive MBE depositions of n-doped alayers, as required for the fabrication of a particular photodiode ortransistor structure. However, the necessity for multiple, successivedepositions of CdTe/HgTe homogeneous pairs within each p-type layer, andsubsequent annealing of the structure, is obviated.

The present invention thus entails several technical advantages overprior techniques employed for p-type doping of Group II-Group VImaterials. These include the fact that the steps required to produce ap-doped II-VI structure are substantially fewer than those required inprior art methods employing multi-layer diffusion/annealing processes.Further, hetorojunctions are formed with precisely defined, matching n-pjunctions, producing devices of high sensitivity to infrared radiation,and low levels of background signals emitted during "dark" conditions.

BRIEF DESCRIPTION OF THE DRAWING

Other aspects of the invention may be appreciated with reference to thefollowing detailed description, taken in conjunction with the appendeddrawings in which:

FIG. 1 is a schematic representation of an exemplary MBE system inaccordance with the invention;

FIG. 2 is a schematic cross-sectional view of a structure formed by themethod and apparatus of the present invention;

FIG. 3 is a schematic cross-sectional view of a photodetector deviceformed by the method and apparatus of the present invention; and

FIG. 4 is a schematic cross-sectional view of a three-layer deviceformed by the method and apparatus of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

While the invention will be described with reference to an exemplarymolecular beam epitaxy growth apparatus and with respect primarily tothe formation of p-doped layers of particular Group II-Group VImaterials, it should be understood that the invention is also applicablefor use with other forms of molecular beam epitaxy apparatus and is alsoapplicable in the formation of a variety of Group II-Group VI binary andthree-layer semiconductor structures.

With initial reference to FIG. 1, an exemplary molecular beam epitaxialgrowth apparatus 10 in accordance with the present invention is shown,suitably including a commercially available MBE growth chamber such asthe Riber Model 2300-P system, modified and configured in accordancewith the present invention as will be described hereinbelow. Theconstruction and operation of such MBE apparatus is generally known bythose in the art, e.g., as disclosed in U.S. Pat. Nos. 4,605,469 and4,622,083, which are hereby incorporated by reference. Accordingly, thedetails of construction of the MBE apparatus will not be described indetail herein.

In summary, the apparatus 10 defines a growth chamber 12 which isadapted to be evacuated to ultra-low pressures by a high vacuum pump 14.A substrate holder suitably including a molybdenum support plate 16 ispositionable centrally within the chamber 12 on a rotatable shaft 18which is connected to and extends perpendicularly from the back surfaceof the support plate. The shaft 18 is axially rotatable by means ofdrive means, such drive means being known to those in the art, andfiguratively represented at 20, the drive means and shaft 18 beingsupported on a positioning member 22 which is axially rotatable fordiverting the support plate 16.

The MBE apparatus 10 is operable for epitaxially growing p-doped layersof various Group II-Group VI semiconductor alloys and compounds.Representative Group II-Group VI structures which are successfully grownand p-doped in accordance with the invention include cadmium telluride,mercury telluride, and Hg_(1-x) Cd_(x) Te. Other suitable Group II-GroupVI materials include CdSe, HgSe, HgCdSe, PbTe, and PbSe.Mercury-containing layers are advantageously utilized in the fabricationof narrow bandgap infrared detectors and emitters, and the method andapparatus of the present invention are adapted for forming p-dopedmercury-containing layers, and various combinations of heterojunctionstructures, in which the devices are formed with extremely precise p/njunctions, with little diffusion of the Group V dopant into any adjacentlayer. As will be understood from the description to follow, the growthand concurrent p-doping of an entire Group II-Group VI layer isaccomplished during a single, continuous epitaxial growth procedure, incontrast to the multiple deposition and diffusion/annealing stepsemployed in previous methods. With reference to FIG. 1, exemplaryembodiments of the process and method of the present invention will nowbe described in which a layer of p-Hg_(1-x) Cd_(x) Te, of sufficientthickness for use in an infrared-sensitive photodiode or the like isepitaxially grown while an activated nitrogen dopant is simultaneouslyintroduced into the epilayer for doping the Hg_(1-x) Cd_(x) Te.

The apparatus 10 includes a plurality of flux sources provided in one ormore effusion cells communicating with the MBE chamber 12. In accordancewith practices known in the art, the effusion cells include cruciblescontaining respective elements or combinations of elements selected forMBE deposition of a particular structure and are individually heated byelectrical power from a power source, not shown, to produce vapor fluxesfrom their respective materials. In operation, the fluxes are directedinto the MBE chamber 12 and toward substrate support plate 16 and anysubstrate mounted thereon. Whereas the number and contents of theeffusion cells vary in accordance with the particular materials beinggrown, in the present, illustrative embodiment in which Hg_(1-x) Cd_(x)Te is to be formed, a first effusion cell 26 of a CdTe compound, and asecond effusion cell 28 of tellurium, are employed. A mercury cell 30 isalso provided, in which liquid mercury substantially fills a chamber 32,which is connected by means of tubing 33 and guide tube 34 to theinterior of the MBE chamber 12. The CdTe and Te effusion cells 26, 28are in communication through respective guide tubes 36, 38 with the MBEchamber 12, the guide tubes being directioned centrally or toward themolybdenum support plate 16, and any substrate thereon.

Further in accordance with the present invention, a nitrogen injectionapparatus 42 is provided for introducing activated nitrogen into the MBEchamber 12 during epitaxial growth of a Group II-Group V layer. Nitrogeninjection apparatus 42 is operable for directioning a stream or jet ofactivated nitrogen along a constrained, substantially linear path towardany substrate supported on the molybdenum support plate 16. The nitrogeninjection apparatus 42 includes an ionization means for convertingmolecular nitrogen (N₂), received from molecular nitrogen source 44,into activated, atomic nitrogen for injection into the MBE chamber 12.The molecular nitrogen contained in source 44 is extremely pure. Forexample, nitrogen of 99.9999 percent purity has been successfullyemployed. In the preferred embodiment, the nitrogen injection apparatus42 includes an electron cyclotron resonance (ECR) plasma source 46, suchas those manufactured by Wavemat. The ECR plasma source 46 includes anelongated, generally tubular housing 48 having an inlet end connectedfor receiving the molecular nitrogen 44 and an outlet or downstream endconnected to an elongated guide tube 50. The elongated guide tube 50 ispreferably quartz-lined, as shown at 52, and is connected between plasmachamber 54 defined within the ECR housing 48, and the MBE chamber 12.The guide tube 50 is preferably directioned toward the molybdenumsupport plate 16. Preferably, the robe 50 is sufficiently long that itextends within the MBE chamber 12 for a substantial distance toward thesubstrate, wherein the spacing 56 between the electron cyclotronresonance device housing 48 and the distal end of the guide tube 50 isgreater than the distance, indicated at 58, between the support plate 16and the distal end portion of the guide tube 50. Preferably, the lengthof the elongated guide tube 50 is twice that of the spacing 58 betweenthe distal end portion thereof and the substrate support plate 16.Further, the inner diameter of the quartz lining 52 is small relative tothe length 56 of the tube 50, whereby the nitrogen is injected as anarrow stream, and wherein its angle of dispersion is quite small. Theelongated, quartz-lined guide tube 50 thus provides several technicaladvantages in the operation of the system. The relatively small innerdiameter of the quartz lining 52, e.g., suitably of about 13/8 inches,in combination with the extended length of the tube 50, e.g., of about 8to 15 inches, serves to direction the activated nitrogen toward thesupport plate 16 and, further, to minimize any influx of mercury vaporfrom the chamber 12 into the plasma chamber 54 during the MBE depositionprocedures. It has not been previously recognized that the length ofsuch a guide tube is of any criticality, or that the angle of dispersionof a p-type dopant is of importance in permitting the use of anactivated nitrogen dopant in such MBE processes, as will be more fullydiscussed hereinbelow.

The electron cyclotron resonance housing 48 is connected via a controlvalve 60 to the nitrogen source 44. When energized, such ECR plasmagenerators create a concentrated magnetic field, indicated at 62, forplasma confinement, and a microwave power supply input device 64 isprovided through which microwave energy, e.g., suitably at 2.45megahertz, is introduced into the plasma chamber. Such devices serve toprovide a magnetic field in which the modulus of the magnetic field 62is minimal near the center of the chamber 54, wherein the Larmorfrequency of the electrons defined by the magnetic field equals thefrequency of the injected microwaves. Electrons crossing the field arethus energized by the electron cyclotron resonance. Accordingly,although molecular nitrogen (N₂) is in general very unreactive, becauseof the strong triple bond between the N₂ atoms, when activated by such aplasma source, substantially all of the nitrogen is converted toactivated, atomic nitrogen, which is highly reactive. The use of such anelectron-cyclotron-resonance plasma source is preferred over otherplasma source devices because of its efficiency and its capability ofoperation at very low pressures, of about 1×10⁻⁴ or 10⁻⁵ Torr, which arecompatible with those within the MBE chamber 12. However, other plasmasources, such as those of the radical-beam type, may also be employed.During MBE growth process, the relatively long, quartz-lined guide tube50 buffers the plasma source 46, whereby the plasma chamber 54 isdistanced and substantially isolated from the MBE chamber 12, therebyminimizing mercury absorption within the plasma chamber 54.

To summarize the operation of the ECR plasma source 46 during themolecular beam epitaxial growth process, the ECR plasma source 46 isinitially activated, and valve 60 is opened to permit high puritynitrogen (N₂) gas to pass through the valve and the ECR plasma chamber54 to form activated nitrogen, comprising mostly atomic nitrogen, whichis directed through the quartz liner of guide tube 50 toward acrystalline substrate 66 mounted on the substrate support plate 16during epitaxial growth of a Group II-Group VI layer, for effectingp-type doping of the epitaxial layer being grown on the substrate. Asseen in FIG. 2, the p-type Group II-Group VI layer 68 is thereby grownon the outer surface of substrate 66. The doping level is controlled byadjustments of the nitrogen pressure and flow rates, by adjusting valve60. During the MBE growth process, the substrate support plate 16 andsubstrate 66 are preferably rotated by shaft 18 at a constant speed,whereby the substrate is evenly heated by radiation from non-rotatingheater element 70. As stated above, the MBE growth of such GroupII-Group VI materials in general is known in the art, e.g., as discussedin the article of R. J. Koestner and H. F. Schaake, Journal Vac. Sci.Technology, A6, p.2834 (1988), which is hereby incorporated byreference. In one embodiment, to be described in further detail, in alater section, an additional effusion source 72 and guide tube 74 areprovided for emitting an n-type dopant, such as indium, into the MBEchamber 12 prior to, or subsequent to, the formation of the p-typelayer, for forming two layer or three layer devices.

The method of the present invention will now be described with respectto the formation of a p-HgCdTe layer 68, as illustrated in FIG. 2.Initially, a compatible crystalline substrate 66 (FIG. 2) is mounted onthe support plate 16. For example, in the formation of Hg_(1-x) Cd_(x)Te, substrates formed of CdZnTe<100>, CdZnTe<211>-Te, and CdZnTe<111>-Teare suitable. A four percent zinc component is included for latticematching. Suitably, a CdZnTe substrate 66, of approximately 500 μmthickness, is mounted on the support plate 16 by means of a galliumsoldering process, in accordance with methods well known in the art. Forconvenience, the substrate 66 and support plate 16 may be initiallyloaded in a loading chamber (not shown) positioned to one side of theMBE vacuum chamber 12, as described in U.S. Pat. No. 4,605,469,previously incorporated by reference. The loading chamber is thenevacuated and the substrate 66 and support plate 16 are subsequentlytransferred to the evacuated MBE chamber 12 by means of a substratetransfer mechanism, not shown, as also disclosed in U.S. Pat. No.4,605,469.

The MBE chamber is initially baked to remove impurities and oxides fromthe interior surfaces of the chamber 12 and from the guide tubes 50, 36,38, 34, and 74. The MBE chamber 12 is evacuated to a pressure ofapproximately 10⁻¹⁰ Torr by means of high vacuum pump 14. Next, theCdTe, Te, and Hg cells are heated to their respective operatingtemperatures. During the initial heating of the CdTe and telluriumcells, their respective shutters are closed. Additionally, there isinitially no mercury in the mercury cell, whereby the MBE chamber 12remains free of fluxes of the epitaxial materials.

The substrate/support plate assembly 16 is then transferred into the MBEgrowth chamber 12, power is applied to heating element 70, and rotationof the substrate support plate 16 on shaft 18 is initiated as thesubstrate 66 is heated to about 300° C. by the heating element 70 toremove oxide from the CdZnTe substrate 66. The support plate 16 andCdZnTe substrate 66 are then permitted to cool to a growth temperatureof approximately 180° C. During this initial 300° C. baking procedure,the support plate 16 is diverted downwardly, i.e., the positioningmember 22 is rotated axially in the direction shown by arrow 76, wherebythe substrate front surface is preferably deviated downwardly from thegenerally horizontal plane in which guide tubes 36, 38, 34, 50, and 74are aligned, whereby any contaminants in the chamber 12 are notpermitted to fall upon the surface of the substrate 66. When thesubstrate 66 reaches the desired growth temperature, e.g., 180° C.,control valve 60 is opened to permit nitrogen gas to flow from source 44through the ECR plasma source 46, and through the elongated, quartzlined guide tube 50 into the MBE growth chamber 12. The nitrogenpressure in the growth chamber is adjusted by adjusting the rate of flowof the nitrogen with respect to the pumping rate of the vacuum pump 14.Adjustments in the flow rate are suitably made by performing tests,prior to the epitaxial growth of a p-doped layer, for measuring thepressure increase after the valve 60 is opened. As suggested above, thevacuum within the chamber is initially about 10⁻⁹ Torr, and, typically,rises to about 3-4×10⁻⁵ Torr during injection of the nitrogen. Duringthe initial pressure adjustment phase, the ECR plasma source is notactivated.

Upon the substrate 66 reaching a desired epitaxial growth temperatureand the rate of nitrogen flow being adjusted, positioning member 22 isaxially rotated to bring the substrate 66 into alignment with the guidetubes 34, 36, 38, 50 and 74. Next, mercury is allowed to enter the MBEgrowth chamber 12 by adjustment of a valve, not shown, in line 33, or byother suitable flow control means. When the mercury flux has beenadjusted, the CdTe and tellurium effusion cell shutters are opened, andthe electron cyclotron resonance plasma source apparatus 46 is turned onfor generating the plasma which activates the flow of nitrogen. At thisstage, mercury, tellurium, and CdTe fluxes are continuously applied tothe substrate in association with the activated nitrogen, effecting theepitaxial growth and nitrogen doping of p-Hg_(1-x) Cd_(x) Te. The MBEchamber 12 is maintained at a pressure of approximately 10⁻⁴ Torr duringthis growth process, and deposition rates of approximately 2 μm per hourare typically attained. The resultant structure is illustrated in FIG.2, wherein the CdZnTe substrate 66 is mounted on the substrate supportplate 16 beneath the p-H_(1-x) Cd_(x) Te layer 58, and wherein x is 0.3.

After several repeated runs, mercury will become absorbed on the wall ofthe MBE growth chamber 12. Accordingly, when an excess of mercury hasaccumulated, e.g., after several runs, or on a weekly basis, the chamberis heated to a temperature of about 100° C., and the absorbed mercury isvaporized and transferred to a Hg-collecting chamber, not shown. Duringthis baking procedure, the ECR plasma generator 46 is protected frommercury adsorption by baking it at a temperature of approximately 100°C., suitably by means of resistive heating tapes wrapped around theguide tube 50 and ECR apparatus 46.

The electrical properties of the p-Hg_(1-x) Cd_(x) Te layer 68 aresuitably characterized by Hall measurements. Typically, the p-typedoping levels in p-Hg_(1-x) Cd_(x) Te (x=0.3), and in other mercurycompounds such as HgTe and Hg_(1-x) Cd_(x) Te (x=0.28), are in themid-10¹⁸ /cm³ range. As suggested above, the process is also adapted foruse in the growth of other Group II-Group VI materials. The p-dopinglevels in MBE grown CdTe, for example, are typically in the low-10¹⁹/cm³ range, with room temperature Hall mobilities as high as 130 cm²/V.sec.

The structure illustrated in FIG. 2 is typically utilized as a teststructure for verifying and adjusting the apparatus 10. Referring now toFIG. 3, the process is also advantageously employed for forming binaryor diode devices 86 having p/n junctions. In such a process, anintermediate, n-Hg_(1-y) Cd_(y) Te layer 78 is initially formed over theCdZnTe substrate 66 prior to the formation of a p-doped Hg_(1-x) Cd_(x)Te layer 80, in accordance with the method discussed above. The MBEgrowth of n-doped Group II-Group VI layers is accomplished by theabove-described MBE apparatus 10 and growth procedure, except thatindium cell 72, rather than the nitrogen injection apparatus 42, isactivated during the process of growing the n-type layer 78. Thus, anindium dopant is incorporated into layer 78 during its growth, resultingin n-type dopant characteristics in the resultant n-Hg_(1-y) Cd_(y) Te(y=0.2) layer 78, after which p-doped layer 80 is grown on the n-typelayer utilizing the nitrogen injection apparatus 42 as discussed above.Thus, a p-n diode structure 86 is formed, as shown in FIG. 3. The use ofindium as an n-type dopant for HgCdTe during MBE processes for formingGroup II-Group VI structures is known in the art (see, e.g., J. M.Arias, et al., J. Vac. Sci. Technol. A8, pp. 1025-1033 (1990)), and willnot be described in detail herein. However, it has not been previouslyrecognized that Group II-Group VI device structures may be epitaxiallygrown in an MBE chamber in an ongoing process wherein the dopantcharacteristic may be changed from n-type to p-type by closing theshutter of an effusion cell of n-type dopant and energizing an ECRplasma generator or other plasma source for injecting an activatednitrogen p-type dopant, continuing the growth procedure without removingthe substrate from the MBE chamber 12.

The apparatus 10 is thus readily adapted for forming a variety ofdevices, with electrical characteristics corresponding to respectivedevice specifications. For example, and with reference now to FIG. 4, ann-p-n trilayer device 88 is conveniently formed by the apparatus 10. Then-p-n transistor device 88 suitably includes a base n-type layer,similar to n-type layer 78 of FIG. 3, an intermediate p-doped layer 92,similar to p-type layers 80 of FIG. 3 and 68 of FIG. 2, and an upper,n-doped layer 94 similar to layers 90 and 78. The three layer devicestructure 88 is formed in accordance with the process described abovewith respect to the formation of the two-layer device 86 of FIG. 3,followed by an additional MBE deposition step in which the nitrogeninjection apparatus 42 is shut off, valve 60 being closed, and theshutter of indium effusion cell 72 is opened, cells 26, 28 and 30 alsoremaining open. Again, the device structure 88 is formed during acontinuous MBE growth process without vacuum release or removal ofsubstrate 66 from MBE chamber 12.

It may now be understood that the process and apparatus of the presentinvention afford significant technical advantages over prior-art methodsand apparatus. By the use of an activated nitrogen dopant in associationwith the MBE growth of a Group II-Group VI layer, the problems inherentin prior-art doping systems employing fast diffuser elements such asarsenic are substantially eliminated, since the nitrogen atom is a veryslow diffuser in such materials. That is, because the nitrogen atom doesnot diffuse rapidly, the nitrogen dopant effused into the chamber 12 andthe Group II-Group VI layer does not tend to diffuse into any adjacentn-doped layer, and, accordingly, precise heterojunctions betweenadjacent p-type and n-type layers are achieved, thereby greatlyenhancing the performance of photodiodes constructed in accordance withthe present method.

A further technical advantage of the invention with respect to prior-artprocesses in which fast diffusing dopants such as arsenic are used isthat the requirement for successive deposition and annealing procedureswith respect to multiple pairs of doped and undoped layer pairs, andsubsequent annealing of the structure, is eliminated, in that theprocess of the present invention is adapted for growing p-doped GroupII-Group VI layers in a continuous, uninterrupted process, and in anydesired thickness.

Additionally, the use of nitrogen as a dopant entails the furtheradvantage that it is substantially less toxic than arsine or itsderivatives. Further technical advantages are entailed in theutilization and construction of the plasma source, and the elongatedguide tube employed for directing the activated nitrogen in a narrowbeam toward the CdTe substrate and for minimizing any return influx ofmercury or other volatile elements from the MBE chamber. Thus, suchelements are substantially prevented from entering and depositing on theinterior of the plasma source, thereby obviating the need for frequentand time consuming bakeouts of the plasma source, e.g., bakeouts aftereach deposition, thus greatly increasing the throughput of the apparatus10. As discussed, it will now be apparent to those in the art thatvarious combinations of elements may be used, and that various devicestructures, including two-layer and three-layer device structures, mayreadily be formed using the process and apparatus of the presentinvention. It will further be apparent that because of the preciselycontrollable MBE growth processes, layers of precise and differingthicknesses may be readily formed.

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thisdescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method for forming a p-doped semiconductorlayer, comprising the steps of:providing a crystalline substrate;supporting the substrate within a chamber; epitaxially growing on thesubstrate a semiconductor layer consisting essentially of Group II andGroup VI materials with mercury included in the Group II material, whileintroducing activated nitrogen into the chamber to effect p-type dopingof the semiconductor layer.
 2. The method of claim 1, wherein the stepof epitaxially growing a semiconductor layer comprises introducing aflux of Group VI materials selected from one or more of the groupconsisting of tellurium and selenium.
 3. The method of claim 1, whereinthe step of epitaxially growing a semiconductor layer on the substratewhile introducing activated nitrogen into the chamber comprises thesteps of:providing a source of nitrogen in molecular form: providing aplasma source connected between the source of molecular nitrogen and thechamber and causing the nitrogen to flow through the plasma source foractivating the molecular nitrogen; and injecting the activated nitrogeninto the chamber.
 4. The method of claim 3, wherein the plasma sourcecommunicates with the chamber through a buffering means for isolatingthe plasma source from the chamber, the buffering means defining achannel, and wherein the step of injecting activated nitrogen comprisesinjecting activated nitrogen into the chamber through the channel. 5.The method of claim 4, wherein the buffering means comprises a guidetube which projects within the chamber, and wherein the step ofinjecting activated nitrogen into the chamber comprises passing thenitrogen through the guide tube for a distance which is greater than thedistance between the guide tube and the substrate.
 6. The method ofclaim 1, further comprising epitaxially growing another semiconductorlayer on the substrate while effecting in-situ n-type doping of saidanother semiconductor layer.
 7. A method for forming a semiconductordevice, comprising the steps of:providing a crystalline substrate;supporting the substrate within a chamber; epitaxially growing a first,n-doped semiconductor layer on the substrate; epitaxially growing asecond semiconductor layer on the first layer, the second layerconsisting of at least one Group II element and including mercury and atleast one Group VI element selected from the group consisting oftellurium and selenium; and introducing activated nitrogen into thechamber, during the step of epitaxially growing the second layer andeffect p-type doping of the second layer.
 8. The method of claim 7,wherein the step of epitaxially growing the first semiconductor layercomprises introducing a flux consisting essentially of the Group II andGroup VI materials of the second layer while introducing a flux of ann-type dopant.
 9. The method of claim 6, further comprising:epitaxiallygrowing a third, n-doped semiconductor layer on the second, p-dopedsemiconductor layer.
 10. A method for epitaxially growing a p-typesemi-conductor layer consisting essentially of Group II and Group VImaterials, comprising the steps of:providing a crystalline substrate;supporting the substrate within a molecular beam epitaxy chamber;growing a semiconductor layer on the substrate by molecular beamepitaxy, by introducing flux into the chamber consisting essentially ofat least one Group II element and including mercury and at least oneGroup VI element selected from the group consisting of tellurium andselenium; and introducing activated nitrogen into the chamber, duringthe step of epitaxially growing a semiconductor layer to effect in-situp-type doping of the semiconductor layer.
 11. The method of claim 10,wherein the at least one Group II element comprises mercury and cadmium.12. The method of claim 11, wherein the at least one Group VI elementcomprises tellurium.
 13. The method of claim 12, wherein the step ofintroducing activated nitrogen into the chamber comprises the stepsof:providing a source of molecular nitrogen and providing activatingmeans connected between the molecular nitrogen source and the chamber;activating the molecular nitrogen by causing it to flow through theactivating means; causing activated nitrogen to flow from the activatingmeans to the chamber along a passageway directioned toward thesubstrate.
 14. The method of claim 13, wherein the activating meanscomprises an electron cyclotron resonance means for generating aresonant magnetic field, and wherein the step of activating themolecular nitrogen comprises causing the molecular nitrogen to flowthrough the magnetic field.